Heat shield for spark plug cables



Aug. 15, 1939. F. G. GARDNER HEAT SHIELD FOR SPARK PLUG CABLES Filed Feb. 24, 1934 ATTOR N EY Patented Aug. 15, 1939 PATENT OFFICE HEAT SHIELD FOR SPARK PLUG CABLES Frank G. Gardner, Maplewood, N. J assignor to Breeze Corporation, 1110., Newark, N. J.

Application February 24, 1934, Serial No. 712,696

Claims.

This invention relates to a heat shield.

The invention is adapted for use with any spark plug shield or spark plug conductor, but for the purpose of this description the invention is de- 5 scribed as applied to one form of spark plug shield commonly used on aircraft engines.

In the operation of internal combustion engines considerable heat is generated in the cylinder head adjacent the spark plug and in the spark plug proper.. By reason of the contact with the spark plug and the proximity of the cable to the cylinder head, an appreciable amount of such heat is transmitted to the spark plug cable. Spark plug cables usually comprise a strand wire conductor covered with insulation. The heat transmitted to the cable causes charring of the conductor and the insulation, and tends to cause a rapid deterioration of the insulation or a breakdown of the conductor. By reason of this condition the service life of a spark plug cable is relatively short, and it is necessary to frequently replace the cable in order to prevent cable failure in flight due to a breaking down of the insulation or the burning of the conductor. Therefore, one object of this invention is to substantially reduce the heat transmitted to the spark plug cable from the spark plug and cylinder head by impeding the passage of the heat through the media by which such heat usually reaches the cable.

In aircraft practice it is common to provide radio shields for the spark plugs. Such shields generally comprise a metallic cup to surround the plug, together with a metal covered dielectric cap to cover the plug in combination with a metal conduit to cover the cable, as more fully shown in co-pending applications Serial Nos. 481,766; 550,076; 657,166, and 657,167. Where separate radio shields are not used it is common to use radio shielded spark plugs wherein the radio shielding is made as an integral part of the plug. Such shielded plugs may be of the type shown in co-pending application Serial No. 358,711.

Two separate forms of the invention are described and shown herein. Where shielded plugs or plugs without shielding are used, one form of the invention may be used to protect the spark plug conductor herein referred to as the spark plug cable. Where separate spark plug shields are used, either form of the invention may be used to protect the dielectric shielding cap and the spark plug cable. It is not necessary to simultaneously use both forms of the invention. Either form is effective for the purpose of the invention, that is, to so reduce the heat transmitted to the dielectric shielding cap and/or the spark plug cable that such heat as may be transmitted cannot per se cause damage. 7

Where separate spark plug shields of the type 5 described in the above mentioned co-pending applications are used, heat from the spark plug and cylinder head reaches the dielectric cap and the cable in three ways. Some heat is radiated through the air inside the shield to the dielectric cap over 10 the plug and thence to the cable. Some heat is transmitted by direct conduction from the electrode of the spark plug to the cable through the metal contact means. A considerable portion of the heat is transmitted by conduction through 15 the metal cup surrounding the plug to the metal covered dielectric cap over the plug and thence through the cap to the cable. Some heat may be transmitted by radiation outside of the shield from the cylinder head to the conduit enclosing 20 the cable. However, the amount of heat thus transmitted is negligible and would have no harmful eifect on the cable. Therefore, the specific object of this invention is to provide means to impede the transmission of heat from a cylin- 25 der head and spark plug to the shielding cap and the spark plug cable where the spark plug is enclosed in a metallic radio shield.

Many engines in present use are equipped with radio shielding systems including spark plug 30 shields or shielded plugs of the type above described and shown in the above mentioned copending applications. It is desirable to provide a heat shield which may be used in conjunction with existing spark plug shields or shielded plugs. 35 Therefore, a further object of the invention is to provide a simple heat shield which may be easily and quickly applied to existing spark plug shields or shielded spark plugs without the necessity of replacing the entire spark plug shield or plug.

A further object of the invention is to provide means to dampen or suppress improper electrical emanations from the spark plug cable.

The invention consists of the construction, combination and arrangement of parts, as herein illustrated, described and claimed.

In the accompanying drawing, forming part hereof, are illustrated two forms of the invention, in which drawing similar reference characters designate corresponding parts, and in which letters are used to indicate well known parts and numerals used to indicate the structure comprising the invention, and in which the figure is a vertical section, partly in elevation, showing a conventional spark plug with the invention applied thereto.

Referring to the drawing, A designates a cylinder head in which is inserted a spark plug B. Disposed around the base of the plug B is an open-ended metal cup I. The cup I may be made of any suitable metal and it is common to make shielding cups of an aluminum alloy. The cup I is provided at its bottom with an inturned flange 2 adapted to seat on the cylinder head A. A metal washer W may be placed betwen the flange 2 and the base of the spark plug B. In this way the cup I is rigidly secured to the cylinder head A by the plug B. Near the top of the cup I but removed from the edge thereof is provided an angular offset seat 3, and the upper edge of the cup I is turned over to provide a shoulder 4 in which is disposed a re-inforcing L-shaped ring 5.

Disposed in the cup I is a cylindrical metal shield 6. The lower edge of the cylindrical member 6 is formed with a downwardly and inwardly projecting bafiie I adapted to seat on the offset 3 of the cup I. The center of the baflie I is provided with an opening 8 to permit the application of the member 6 over the spark plug B, and the bafile 1 is adapted to make a relatively close fit with the spark plug B at the base of the exposed porcelain or other dielectric portion of the plug B.

The member 6 with the bafiie I is made of a metal having relatively poor heat conducting properties but relatively good electrical conducting properties. Chrome-nickel alloys have poor heat conducting properties. Any one of several well known chrome-nickel alloys may be used. The chromium content is the principal factor in heat conductivity. As the percentage of chromium is increased the heat conductivity of the alloy is decreased. For example, nichrome, which is the trade name for a nickel alloy steel having approximately a 30% chromium content, has been found most efiicient for use as a heat shield. Since it is desirable to reduce the heat only to a point below a critical temperature where the heat might cause charring or other damage, chrome-nickel alloys with a lower chromium content than nichrome may be used with satisfactory results.

Disposed over the spark plug B is a dielectric cap C having a metal covering D. At the base of the cap C is provided an annular recess E.

The cap C is of the type in common use and the recess E ordinarily receives a flange on the upper edge of the cup surrounding the plug, as more fully exemplified in co-pending application Serial No, 657,166. The upper end of the chromenickel alloy member 6 is turned over as shown at 9 and frictionally held in the recess E. In the assembly of the shield using this invention the edge 9 of the member 6 is first engaged in the recess E and then the cap C is placed over the plug B. The cap C is held in place by means of a bail R pivotally carried by the shoulder 5 of the cup I. A ball S is provided on the bail R so that the ball may be easily removed from or engaged with the cap C.

The structure thus far described is completely efiective for the desired purpose. The baffle I prevents direct radiation inside the shield. Since the bafile I is of poor conducting quality a relatively small amount of heat will pass therethrough to be radiated from the upper side of the bafile I. At the same time, the cylindrical member 6 impedes the direct conduction of heat from the cup I to the cap C with its metallic covering D. However, a complete radio shield is nevertheless provided so that any improper electrical emanations from the spark plug B are conducted to ground. In addition to preventing an appreciable amount of heat from reaching the spark plug cable, it will be clear that this construction reduces the heat normally transmitted to the shielding cap and thereby increases the service life of the cap itself, as well as the spark plug cable.

Openings -2Il may be provided in the cup I to assist in cooling the spark plug B, and if such openings are not too large they will not afiect the radio shielding qualities of the cup I. With this construction the only appreciable heat which may reach the cap C or the spark plug cable is by direct conduction through the electrode of the spark plug B. However, by reason of the decrease in the heat transmitted by direct radiation, and by conduction through the spark plug shield, the heat transmitted through the electrode is insuflicient to cause damage to the cable. As pointed out hereinafter, the converse is also true, If direct conduction of heat through the electrode of the plug B is materially reduced the heat transmitted by direct radiation within the shield and by conduction through the shield, where the chrome-nickel member 6 with the baffie I is not used, is insufficient to cause damage to the cap C or the spark plug cable.

It will be clear that the members heretofore described may be easily applied to engines presently equipped with shielding. In order to apply the device it is only necessary to release the ball R, remove the cap C, and remove the plug B. The

standard shielding cup may then be removed,

and the cup I with the heat shield 6 substituted therefor.

The cap C is provided with an extension F, which extension is provided with a bore G to receive a spark plug cable. Set in the cap C is a bushing H through which is disposed a pointed contact screw J having afiixed to the lower end thereof a spring K for making contact with the upper end of the plug B. Considerable heat is transmitted to the cap C by direct conduction from the electrode of the plug B through the spring K, bushing H and contact screw J. By making these three elements of a chrome-nickel alloy the cap C is protected from overheating.

To further protect the spark plug cable, a rod of a chrome-nickel alloy I0, molded in a dielectric sleeve II, is disposed in the opening G in contact with the screw J. The opposite end of the rod E6 is provided with a contact face I 2 adjacent which is disposed a spring I3. The rod III with its sleeve II projects past the end of the extension F on the cap C. The rod III with the sleeve II extend into a recess I5 of a second dielectric sleeve It. The opposite end of the sleeve I4 is provided with a bore I1 to receive the cable which comprises a strand wire conductor M surrounded by insulation N. The conductor M is passed through an orifice I6 in the center of the center of the sleeve I I and the strands of wire spread to make contact with the spring I3. The spring I3 may be provided with a contact washer, but if a permanent connection is desired a drop of solder may be disposed on the ends of the strands forming the conductor M to hold them in spread position in order to make proper contact.

Beyond the lower extremity of the sleeve If,

the conductor M with its insulation N, is inclosed in a shielding conduit 0 provided on its end with a ferrule P having a flange Q. The end of the sleeve I4 is provided with a tapered recess into which is forced a conical washer I9, and a coupling nut i8 is used to secure the assemblage to the extension F of the cap C. A conical washer L is disposed in a tapered recess at the extremity of the extension F so that a water-proof joint is prohibited at each end of the coupling nut l8.

The rod l0 serves to complete the electrical connection from the conductor M at the orifice 16 to the contact screw J adjacent the top of the spark plug B. By reason of the low conductivity of the rod l0 very little heat may be transmitted to the conductor M or the insulation N from the electrode of the spark plug B. Where the rod I0 is used without the shield 6 the rod I0 is alone effective to reduce the heat transmitted to the cable. In order for heat from the shield to affect the cable it is necessary for the heat to pass not only through the cap C, but also through the extension F, the washer L, the sleeve l4 and the nut 18. By reason of the fact that the cable is so far removed from the source of heat the total amount of heat reaching the cable from the shield and through the rod III is insufficient to cause charring with its attendant danger.

In present shielding systems where this invention is not used the conductor M with the insulation N extends to the uppermost limits of the opening G. The contact screw J is used to pierce the insulation N to make contact with the conductor M. Under these conditions the ferrule P and the conduit 0 abut the washer L, and a short coupling nut is used to secure the assemblage together. It can therefore be easily seen that the rod l0 and its associated parts may be readily applied to existing shields. It is only necessary to shorten the cable by cutting a section therefrom and shortening or substituting a shorter conduit 0. If the operation is to be performed in the field it is quite simple to cut a short section from both the cable and the conduit 0, and a new ferrule P may be readily applied by the use of the hand tool for that purpose, as described in copending application Serial No. 672,917.

Where the rod in is used the efficiency of the radio shielding system is increased. The conductor M carries high tension current. Ignition leads release electrical emanations which interfere with radio equipment. 4 This is particularly true of high tension conductors such as the conductor M. For this reason radio shielding is 1. A heat shield for spark plug cables comprising a sleeve of metal of low heat conductivity adapted to be disposed around a portion of a spark plug and spaced therefrom and formed with a battle disposed between the sleeve and the body of a spark plug.

2. A. heat shield for spark plug cables comprising a sleeve of metal of low heat conductivity adapted to be disposed around a portion of a spark plug and spaced therefrom and formed with a baffle disposed between the lower edge of the sleeve and the body of a spark plug.

3. A spark plug radio shield adapted to enclose a spark plug and having a section thereof formed of a metal of low heat conductivity to restrain the transmission of heat from the lower portion of the shield to the upper portion thereof.

4, A spark plug radio shield adapted to enclose a spark plug having a section of its wall made of a metal of low heat conductivity to impede conduction of heat through the shield from the lower portion thereof to the upper portion thereof.

5. A spark plug radio shield adapted to enclose a spark plug having a section of its wall. made of electrically conductive material of low heat conductivity to impede conduction of heat through the shield to the upper portion thereof.

6. A spark plug radio shield adapted to enclose a spark plug having a section of its Wall made of a metal of low heat conductivity to impede conduction of heat through the shield from the lower portion thereof to the upper portion thereof, said section having a baffle formed thereon to restrict radiation of heat within said shield.

7. A spark plug radio shield adapted to enclose a spark plug having a section of its wall made of chrome-nickel alloy to impede conduction of heat through the shield from the lower portion thereof to the upper portion thereof, said section having a battle formed thereon to restrict radiation of heat within said shield.

8. A combined radio and heat shield for a spark plug comprising an open ended member to surround a spark plug, a cup shaped element formed of a metal of low heat conductivity carried by said open ended member and being formed with an opening in the base thereof to admit a portion of a spark plug and a cable receiving shielding cap disposed over the cup shaped element.

9. A combined radio and heat shield for a spark plug comprising an open ended member to surround a spark plug, a cup shaped element formed of a chrome-nickel alloy carried by said open ended member and being formed with an opening in the base thereof to admit a portion of a spark plug and a cable receiving shielding cap disposed over the cup shaped element.

10. In a radio shield for spark plugs, a cup shaped element of low heat conductivity having an opening in the bottom thereof and adapted to surround a portion of the spark plug.

FRANK G. GARDNER. 

